Today, motors for hybrid vehicles, electric vehicles, home electric appliances such as air conditioners and washing machines, industrial machinery and the like are required to have energy-saving, high efficiency and high performance characteristics. Accordingly, magnets used for such motors are required to have a higher magnetic force (coercive force, saturation magnetic flux density). At present, iron nitride-based magnetic particles are attracting attention as magnetic particles used to form a magnet, and various proposals have been made on such iron nitride-based magnetic particles (see Patent Literatures 1 to 3).
Patent Literature 1 describes ferromagnetic particles which comprise an Fe16N2 single phase, have surfaces coated with an Si compound and/or an Al compound and have a BHmax value of not less than 5 MGOe. The ferromagnetic particles can be obtained by coating the surfaces of iron compound particles with the Si compound and/or the Al compound, followed by reduction treatment and then nitridation treatment. The iron compound particles used as a starting material are composed of iron oxide or iron oxyhydroxide.
Patent Literature 2 describes ferromagnetic particles which comprise an Fe16N2 compound phase in an amount of not less than 70% as measured by Mössbauer spectrum, contain a metal element X in such an amount that a molar ratio of the metal element X to Fe is 0.04 to 25%, have surfaces coated with an Si compound and/or an Al compound and have a BHmax value of not less than 5 MGOe. The metal element X is at least one element selected from the group consisting of Mn, Ni, Ti, Ga, Al, Ge, Zn, Pt and Si.
The ferromagnetic particles are obtained by subjecting iron compound particles previously passed through a mesh having a size of not more than 250 μm to reduction treatment and then to nitridation treatment, the iron compound particles used as a starting material being formed of iron oxide or iron oxyhydroxide which has a BET specific surface area of 50 to 250 m2/g, an average major axis diameter of 50 to 450 nm and an aspect ratio (major axis diameter/minor axis diameter) of 3 to 25 and comprises a metal element X (wherein X is at least one element selected from the group consisting of Mn, Ni, Ti, Ga, Al, Ge, Zn, Pt and Si) in such an amount that a molar ratio of the metal element X to Fe is 0.04 to 25%.
Patent Literature 3 describes ferromagnetic particles comprising an Fe16N2 compound phase in an amount of not less than 80% as measured by Mössbauer spectrum, and each having an outer shell in which FeO is present in the form of a film having a thickness of not more than 5 nm.
The ferromagnetic particles are obtained by subjecting iron oxide or iron oxyhydroxide having an average major axis diameter of 40 to 5000 nm and an aspect ratio (major axis diameter/minor axis diameter) of 1 to 200 as a starting material to dispersing treatment to prepare aggregated particles having D50 of not more than 40 μm and D90 of not more than 150 μm, allowing the obtained aggregated particles to pass through a mesh having a size of not more than 250 μm, subjecting the iron compound particles passed through the mesh to hydrogen reduction treatment at a temperature of 160 to 420° C. and then subjecting the resulting particles to nitridation treatment at a temperature of 130 to 170° C.